1. Field of the invention
The present invention relates to an antenna unit and an accompanying communication apparatus. More specifically, the present invention relates to an improved antenna unit and communication apparatus for optimized use in more than one frequency band.
2. Background
Mobile communication devices that perform radio communications, such as a notebook type personal computers, Personal Digital Assistants (PDAs), etc., need to be as small as possible to maximize consumer acceptance. In addition, such devices must increasingly be capable of efficient communication across a plurality of frequency bands often being used for wireless LANs. Conventionally, for such purposes, a print dipole antenna has been proposed which is shareable between two frequency bands. For more information on such antennas, the reader may refer to the following papers:                Yosio Ehine “Print Dipole Antenna Sharable between Two Frequencies: Non-feed Element Side Arrangement” Proceedings of the 1989 IEICE Spring General Conference B-72, p. 2-72; and        Masatoshi Karigome “Energizing of Non-feed Element in Print Dipole Antenna Sharable between Two Frequencies” proceedings of the 1989 IEICE Spring General Conference B-73, p. 2-73.        
In addition, to minimize the size of the antenna, a method has been disclosed in which an antenna corresponding to a first frequency band and an antenna corresponding to a second frequency band are provided on both sides of a substrate. Such an antenna is described in more detail in Published Unexamined Patent Application No. 2003-8325.
However, such two-frequency print dipole antennas use half wave resonance, so that the size of the antenna must be larger than an antenna utilizing ¼ wave resonance and they also make it difficult to realize acceptable communications performance across a wide frequency band, such as the 5 GHz frequency band specified in wireless LAN standards such as IEEE 802.11a.
It is therefore an object of this invention to provide an antenna unit and a communication apparatus that can solve the above-mentioned problems. This purpose is achieved by combinations of characteristics described in the independent claims appended hereto. In addition, dependent claims appended hereto specify further advantageous embodiments of this invention.